The Gsm Service

By: Ian Ball

In GSM mobile phone system the region to be covered by the service is divided into hexagonal cells which are typically 5 km in diameter. The size depends on the density of traffic and would be larger in sparsely populated areas. At the centre of each hexagonal cell there is a base transceiver station (BS) which sends and receives calls to /from mobile phones within its range. The cell shape is hexagonal as the distance between centres of neighbouring hexagons is constant. This property is useful to ensure easy hand off, i.e., transfer of control of a mobile phone from one cell to an appropriate adjacent cell when the user of the mobile phone moves. The base station has several antennas, a controller and a set of transceivers, one transceiver for each channel assigned to that cell. The Base Stations are all connected to a Mobile Telecommunication Switching Office (MTSO) usually by land lines. If the terrain is unsuitable BS may be connected to MTSO by wireless.

MTSO is also connected to fixed phones through PSTN by a land line.
Fixed line to Mobile
The functions of MTSO are to:
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oEstablish a connection between a fixed phone and a mobile phone via BS and vice versa.
oEstablish a connection between two mobile phones by using their respective BSs.
oAllocate appropriate channels to mobile phones to communicate by informing concerned BS.
oManage handoff of mobile phones from a BS to an appropriate one when a mobile user moves.
oMonitor the calls in progress for facilitating charging for use.
Channel Allocation
Before describing how connection is established between a mobile phone and a fixed phone as well as between two mobile phones we will have to understand how channels are allocated to mobile phones when they want to communicate. The most expensive resource in mobile phone system is the electromagnetic spectrum allocated for this purpose .A specific band of frequencies is allocated to a cell phone company by the government which has to be used judiciously by the company to support as many mobile subscribers as possible. In GSM system this band is divided into a set of sub bands and one set is allocated to each cell for communication within that cell. As mobile phones have a limited power (due to small size and need to conserve battery life) their transmission has a limited range and thus the sizes of the cells are small (a few kms) . Thus a sub band of frequencies allocated to a cell can be reused in other cells which are not in their immediate vicinity. This frequency reuse is a very important feature of the cellular communication system. Now let's see how this connection is established.

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